Neocarzinostatin (NCS) is an acidic antitumor protein with 109 amino acid residues. Amino acid sequence analysis indicated the protein to consist of a single polypeptide chain with two disulfide bonds. Promising clinical results have been reported in Japan in the treatment of gastrointestinal and bladder tumors and in the induction of remission in acute leukemia. Phase I clinical trials will start in this country soon, the Children's Cancer Research Foundation being a participating clinical center. We have prepared a highly purified sample of NCS which is effective against Sarcina lutea and CCRF-CEM human leukemic cells in vitro and L1210 and P388 mouse leukemias in vivo. We now propose to modify the structure of NCS in an effort to improve the antitumor effectiveness and to attempt to identify and "active core" of the molecule. Chemically and enzymatically modified NCS derivatives will be evaluated for biological activity in several in vitro and in vivo systems. Following preliminary work in our laboratory, we plan to prepare radiolabelled NCS of high specific activity for use in the mechanism of action studies and in the development of a radioimmunoassay. Radiolabelled NCS will be used to investigate the interaction of NCS with cell membranes and the transport of NCS into mammalian cells. Inhibition of DNA synthesis and degradation of DNA by NCS will also be studied.